1. Field of the invention
The present invention relates to automatic transmissions, in particular for automobile vehicles, comprising planetary gearsets controlled by friction elements such as clutches and brakes, and usually coupled to a start-up coupling device subject to slip, such as a hydraulic torque converter or a fluid coupling, with or without a lock-up clutch.
2. Description of the Prior Art
Conventional three-speed transmissions of this kind have been known for a long time; they utilize a double planetary gearset, four to five friction elements, and a hydraulic torque converter. Complex four-speed transmissions are also known; they are based on the afore-said conventional three-speed transmissions to which are added a double-ratio single planetary gearset and two friction elements, resulting in assemblies which include one double planetary gearset, one single planetary gearset, six to seven friction elements, together with a torque converter and a lock-up clutch.
These complex four-speed transmissions, however, are of considerable bulk, weight and cost, thus limiting their use to luxury, rear-wheel drive vehicles with longitudinal engines, and making such use very difficult or even impossible on increasingly popular front-wheel drive vehicles with transverse engines. For such vehicles, compact four-speed transmissions have therefore been specifically developed, utilizing a double planetary gearset and five to six friction elements, together with a torque converter and optionally a lock-up clutch.
Such compact four-speed transmissions nowadays tend to replace the conventional three-speed transmissions. Hence, new complex five-speed and occasionally six-speed transmissions are presently being proposed they are based on compact four-speed transmissions as described above to which are added a double-ratio single planetary gearset and two friction elements, as in the case of conventional three-speed transmissions. New assemblies have thus been developed utilizing a total of one double planetary gearset, one single planetary gearset, seven to eight friction elements, together with a torque converter and a lock-up clutch.
These new complex five- or six-speed transmissions then have the same bulk, weight and cost inconveniences as the above-mentioned complex four-speed transmissions which limit their use to luxury, rear-wheel drive vehicles with longitudinal engines. Furthermore, serious restrictions arise in determining suitably spaced gear ratios, thus reducing overall quality of the transmission.
Other solutions have been occasionally proposed, but they are generally as complex, and usually have the same inconveniences.
The object of the present invention is thus a transmission which avoids the inconveniences of the complex transmissions and other solutions described above. This means an altogether compact, multispeed automatic transmission, suitable for front-wheel drive, transverse engine vehicles, which features a minimum number of friction elements, and always offers judiciously spaced gear ratios, with high design flexibility.